A little goes a long way

Large companies can save thousands of dollars by upgrading their instrumentation and collecting data to make the most of every step in a complex multi-step process. A major American chemical supplier recently discovered this firsthand when it adopted radar level measurement devices from VEGA Americas.

Case study

The chemical facility is one of the largest in the world in every respect. In operation for decades, the plant employs over 1000 people and manufactures billions of pounds of chemical products every year.

Unreliable level measurement

The company had a level measurement system in place in its chemical storage spheres; however, the technology was outdated and unreliable, and the company was constantly questioning the accuracy of its readings, resulting in the need for a complete update and overhaul of its existing system.

The plant used a float-based system to measure the level of stored hydrocarbons. Float technology has been in use for a considerable amount of time, due in some part to its easy to use design. Using a cable and pulley, the float is lowered into a stilling well inside the vessel. As the liquid level rises and falls, the float moves along with it. The pulley’s rotation is converted to an analog signal that transmits the level measurement.

Float-based level measurement, while lauded for its simplicity, is not noted for its reliability. The float system contains mechanical parts that wear over time and need constant maintenance and adjustment. Additionally, if the pulley gets damaged, the system will malfunction.

It is common practice for chemical plants to install redundant level systems, especially in applications involving aggressive, potentially dangerous media such as petrochemicals. Having an extra level measurement ensures the facility operates at maximum safety. The company prioritises safety, so in addition to the float apparatus described above, it also used a differential pressure system for level measurement in the chemical spheres.

The differential pressure (DP) instruments calculated the level by subtracting the density of the overpressure from the density of the liquid plus the overpressure. However, the accuracy of the system depended on manually inputting the specific gravity (SG) of the chemicals in the tanks. If the SG was incorrect, the accuracy of the entire system was compromised.

Being at the mercy of unreliable and outdated level measurement technology created two severe circumstances for the petrochemical facility. First, inaccurate data prevented the plant from optimising the efficiency of the system – in a large operation, one improvement can yield thousands of dollars in reduced maintenance costs alone. Second, an overfill as a result of an inaccurate level could be harmful to employees. For these reasons, choosing the right level instrumentation is critical to long term safety and success.

VEGA radar sensors use a non-contact design to measure hydrocarbons.

An accurate solution

The plant decided to use VEGAPULS 63 sensors, part of the VEGAPULS 60 series of radar sensors used for level measurement of bulk liquids and bulk solids, to address the problem. The non-contact devices are known for reliability, even under high pressure and extreme temperatures. The VEGAPULS 63 works well with aggressive liquids, and is also suitable for application with strict hygiene requirements.

It is impossible to overstate the significance of using non-contact radar technology to measure liquid level. Precision is a hallmark of non-contact level radar and, with an accuracy of ±2 mm, the VEGAPULS 63 is no exception, even when put to use against liquids with poor reflective qualities, such as those found in the petrochemical industry. Greater accuracy is important because it allows users to make processes as efficient as possible.

However, accuracy was not the only advantage of using these sensors for this particular application. The plant routinely produces butadiene, ethylene and propylene, and other volatile, aggressive liquids that could potentially damage a level sensor on contact. However, smart product design protects the radar from such dangers. The innovative encapsulated antenna system defends the radar from corrosive liquids, making it reliable for use in storage tanks, process vessels, dosing vessels, and chemical reactors. Additionally, the VEGAPULS 63 is SIL 2 certified, meaning it meets the world’s most rigorous standards for functional safety and quality product design. SIL (safety integrity level) compliance signals a commitment to risk reduction on the part of the manufacturer, and lets the user know that they have made a responsible, safety conscious purchase.

The VEGAPULS 63 can measure aggressive liquids without contact.

A major advantage of using non-contact devices is invulnerability to fluctuating product properties and changing process conditions such as temperature, pressure or heavy vapours. Many measurement products find their signals disrupted by environmental changes or damaged by vapours and condensation, but VEGA sensors are built to measure difficult media in unpredictable conditions. Plus, no contact means no wear and no maintenance, saving users considerable time, effort, and costs.

In any application, configuring radar sensors during installation is critical to long term accuracy. The quality of readings depends on having accurate diagnostic measurements of the vessel. VEGA was careful to obtain these measurements to maximise the functionality of the device.

Once installed, the sensor emits short radar pulses toward the measured liquid, which reflects the wave back to the sensor. The instrument calculates the level using the time it takes for the radar pulses to return and the known height of the vessel. A unique feature of all VEGAPULS 60 series sensors is the self-learning echo processing that makes each radar more reliable over time. VEGAPULS 60 series products have a large measurement memory and they can account for signal attenuation due to buildup. Essentially, the longer the radar is used in a vessel, the quicker it evaluates signals and suppresses false echoes to ensure the accuracy of its level reporting.

Application specifics

The petrochemical plant had a large stilling well in place before VEGA overhauled its level measurement system. Stilling wells are used to produce a more trustworthy level output by guarding against signal disruption from foam, buildup on vessel walls, and foreign objects in the media. However, the existing well was designed for the existing system and could not be optimally paired with the new radar sensors. To combat this circumstance, VEGA Americas sleeved the existing stilling well with a smaller well composed of material that worked more effectively with radar. The new, modern well gave the VEGAPULS 63 a calm, clean column of liquid chemicals to contact, maximising the accuracy of the readings.

One advantage of the design of the VEGAPULS 63 is the lack of a spool piece. This allows users to use a bleed ring that keeps the signal as close to the top of the connecting valve as possible. Emitting the signal so close to the valve minimises signal noise created by the valve itself. To keep the sensor close to the flange, the customer used a simple 3 in. ball valve. This choice has the added benefit of making the radar easy to remove even while the tank was in use.

Overall, this solution was generally more complex than a typical application. Atop the storage tank is a process connection for the original stilling well, a flange connecting the new stilling well, and a second flange, including a ball valve, with the VEGAPULS 63 mounted on to this. Although somewhat complicated, this creative solution was critical in solving the plant’s process problems.

Conclusion

Following the radar sensor installation, the chemical supplier no longer relies on inaccurate and potentially dangerous level measurement systems. Reduced maintenance is saving on time and costs, and the liquid storage tanks are operating with increased efficiency. The new sensors have excelled in this application, despite the poor reflective quality of the stored liquid hydrocarbons and demanding process conditions. With no user intervention, VEGA radars deliver accurate, reliable level measurement. The customer had such a successful experience using the sensors, two additional units were ordered.

Written by Tom Brewer, VEGA Americas, USA. This is an abridged version of an article taken from the April 2016 issue of Hydrocarbon Engineering. Subscribers can view the issue by logging in.